fix nlohmann#600

Instead of calling CompatibleObjectType iterator-range constructor,
first convert json::value_type to CompatibleObjectType::value_type

src/json.hpp

@@ -1046,10 +1046,22 @@ void from_json(const BasicJsonType& j, CompatibleObjectType& obj)
     auto inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
     using std::begin;
     using std::end;
+    using value_type = typename CompatibleObjectType::value_type;
+    std::vector<value_type> v;
+    v.reserve(j.size());
+    std::transform(
+        inner_object->begin(), inner_object->end(), std::back_inserter(v),
+        [](typename BasicJsonType::object_t::value_type const &p) {
+          return value_type{
+              p.first,
+              p.second
+                  .template get<typename CompatibleObjectType::mapped_type>()};
+        });
     // we could avoid the assignment, but this might require a for loop, which
     // might be less efficient than the container constructor for some
     // containers (would it?)
-    obj = CompatibleObjectType(begin(*inner_object), end(*inner_object));
+    obj = CompatibleObjectType(std::make_move_iterator(begin(v)),
+                               std::make_move_iterator(end(v)));
 }
 
 // overload for arithmetic types, not chosen for basic_json template arguments

test/src/unit-constructor1.cpp

@@ -170,6 +170,17 @@ TEST_CASE("constructors")
           CHECK((j2.get<decltype(p2)>() == p2));
         }
 
+        SECTION("std::map<std::string, std::string> #600")
+        {
+          std::map<std::string, std::string> m;
+          m["a"] = "b";
+          m["c"] = "d";
+          m["e"] = "f";
+
+          json j(m);
+          CHECK((j.get<decltype(m)>() == m));
+        }
+
         SECTION("std::map<const char*, json>")
         {
             std::map<const char*, json> o {{"a", json(1)}, {"b", json(1u)}, {"c", json(2.2)}, {"d", json(false)}, {"e", json("string")}, {"f", json()}};